Fix Neutron backend API compatibility and multiprocessing fallback

[apullin]

Summary:
This fixes two issues preventing the Neutron backend from building in sandcastle/build environments:

1. NeutronAtenPassManager API compatibility: The NeutronAtenPassManager class was updated to require a neutron_target_spec parameter, but the call site wasn't updated. This caused a TypeError at runtime. Fixed by passing the already-available neutron_target_spec to the pass manager constructor.

2. Multiprocessing fallback: The Neutron converter uses multiprocessing for isolation when converting models, but this fails in restricted build environments (sandcastle) with an EOFError when trying to create a multiprocessing Manager. Added a try/except block to catch EOFError and OSError and fall back to direct execution of the converter when multiprocessing is unavailable. This maintains isolation in normal environments while allowing builds to succeed in restricted ones.

Both issues were pre-existing on trunk and not caused by recent changes.

Differential Revision: D87252895

[pytorch-bot]

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❌ 1 New Failure, 2 Unrelated Failures

As of commit 8aff4aa with merge base 213d24c ():

NEW FAILURE - The following job has failed:

• pull / unittest-editable / windows / windows-job (gh)
  backends/xnnpack/test/recipes/test_xnnpack_recipes.py::TestXnnpackRecipes::test_all_models_with_recipes

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / linux / linux-job (gh) (trunk failure)
  exir/backend/test/test_lowered_backend_module.py::TestBackendAPI::test_emit_nested_lowered_backend_module
• pull / unittest-editable / linux / linux-job (gh) (trunk failure)
  exir/backend/test/test_lowered_backend_module.py::TestBackendAPI::test_emit_nested_lowered_backend_module

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[meta-codesync]

@apullin has exported this pull request. If you are a Meta employee, you can view the originating Diff in D87252895.

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[robert-kalmar]

@apullin , LGTM, although, don't see the change related to

NeutronAtenPassManager API compatibility: The NeutronAtenPassManager class was updated to require a neutron_target_spec parameter, but the call site wasn't updated. This caused a TypeError at runtime. Fixed by passing the already-available neutron_target_spec to the pass manager constructor.

I checked the executorch OSS code base and did not find the above mentioned API compatibility break. Is is something in your internal code base?

[apullin]

I checked the executorch OSS code base and did not find the above mentioned API compatibility break. Is is something in your internal code base?

This is on an internal SDK, yes. My first time syncing between, so I should have labeled that.